1. Field of the invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, laser printer, or the like, and particularly to a fixing unit incorporated into the image forming apparatus for fixing a toner image on a sheet.
2. Description of the Related Art
In a fixing unit provided in an image forming apparatus for fixing a toner image on a sheet, there is included a heating roller in the form of a cylinder which is internally provided with a heater. The heater is on-off controlled while a surface temperature of the heating roller being detected so that the surface temperature of the heating roller (fixing temperature) can be maintained at a substantially fixed level.
However, the heat released from the heater is sequentially propagated from the air within the heating roller to an inner circumferential surface of the roller, and to an outer circumferential surface of the roller. Accordingly, there is some delay in change in the surface temperature of the heating roller relative to the on-off control of the heater. As a consequence, the surface temperature continues to rise (overshoot) even in the case where the heater is turned off at a fixed temperature. On the other hand, the surface temperature continues to fall (undershoot) even in the case where the heater is turned on at a fixed temperature.
The overshoot and undershoot not only vary in individual fixing units in some degree, but also depend on the ambient temperature and the size of the sheet to be used. Normally, a target temperature based on which the heater is on-off controlled is determined assuming a fixed ambient temperature and a standard size of sheet. However, in the case where the ambient temperature rises or a small-sized sheet is used, the overshoot becomes larger, whereby the sheet is displaced relative to the heating roller due to warping thereof and melting of toner. On the other hand, in the case where the ambient temperature falls or a large-sized sheet is used, the undershoot becomes larger, whereby the following problems will occur: toner is fixed to the sheet in an unsatisfactory manner; toner which is not fixed to the sheet deposits on the heating roller and stains a next sheet.